This research has as its goal the study of the mechanism of induction of gamma interferon synthesis by human thymocytes in vitro. It is based on our original observation that human thymocytes can be induced to synthesize interferon by two agents: a lectin and products of B-lymphoblastoid cell lines. This is the first observation of induction of gamma interferon, a lymphokine, by precursors of T lymphocytes. It is our aim to identify the thymocyte subsets that are induced and to correlate the induction of interferon synthesis with the acquisition of other immune functions characteristic of mature T lymphocytes. T lymphocytes play a crucial role in cellular immune defense, and the recruitment of thymocytes in immune defense against pathogens or tumor cells may be of vital importance. In view of the response of thymocytes to stimuli that cause gamma interferon synthesis, we also plan to investigate whether thymocytes acquire other immune functions characteristic of mature T lymphocytes when interferon synthesis is induced. It is our goal to investigate the immunoregulatory functions of gamma interferon, in particular its effect on T-cell-dependent immunoglobulin synthesis by thymocytes. Furthermore, we continue our investigation of the relationship between interleukin-2 and gamma interferon synthesis and the pharmacologic regulation of gamma IFN synthesis. The mechanism of induction of gamma interferon is of crucial importance for the understanding of one of the factors that influence host response to invasion by tumor cells in lines. (IS)